Water sources of Medicago sativa grown in different slope positions in Yanchi County of Ningxia

Abstract

Aims Alfalfa (Medicago sativa) is considered as an elite forage with high economic and ecological values. In semi-arid areas of Northwest China, the low rainfall is far from satisfying water demand of alfalfa. The northern part of Yanchi County is adjacent to Mu Us desert and has a shallow groundwater table. In this area, groundwater could be the potential water sources for growing alfalfa. Our objective was to investigate the growth performance and potential water sources of alfalfa grown in four slope positions with different altitudes in the bottomland of northern Yanchi County. Method Stable O and D isotope compositions (δO and δD) of different water sources and xylem water were analyzed in April, June, July and August 2013. IsoSource was used to calculate the probable contribution of different water sources to the total plant water uptake. Stomatal conductance, carbon isotope discrimination (ΔC) of whole plant, soil volumetric water content in the 0–300 cm soil profile were also determined. Important findings Slope position had a significant effect on water content in the 0–300 cm soil profile. Highest soil water content was found in the slope position of lowest altitude. The δO-δD coordinates of soil water and plant stem xylem water were distributed on the right of Northwest China local meteoric water line (LMWL), indicating that oxygen and hydrogen isotopic compositions in the water sources of alfalfa were subjected to enrichment due to evaporation. The δO values of soil water in the 0–450 cm profile increased with altitude. Soil water δO values decreased with the depth of soil profile for a given slope position. Soil water δO values in deep profile were similar to those in groundwater, implying that groundwater would move to the upper soil layer via soil capillary. Seasonal fluctuation was observed in soil water δO in the 0–40 cm profile, while soil water δO in 朱林等: 宁夏盐池不同坡位旱地紫苜蓿水分来源 1227 doi: 10.3724/SP.J.1258.2014.00118 profile below 270 cm was stable seasonally. Plant stem xylem water δO value was significantly lower (p < 0.001) in the slope position 1 than in other three slope positions in April, July and August. Highest water utilization rate from deep soil profile (below 270 cm) was recorded for alfalfa grown in the slope position 1 in April, June and July. In August, alfalfa grown in the slope positions 1, 3 and 4 mainly used soil water in the 150–270 cm and 270–450 cm profiles and groundwater; highest dependence on soil water in the shallow profile (0–20 cm) was found in alfalfa grown in the slope position 2. Higher yield, whole plant ΔC value and stomatal conductance were observed in alfalfa grown in the slope position 1 than in other three slope positions. These results suggest that bottomland with lower altitude and shallow groundwater table should be adopted when planting alfalfa without irrigation in the semi-arid areas of Northwest China where average annual rainfall is around 280 mm. Thus, groundwater could contribute to better growth performance of alfalfa, leading to higher ecological and economic returns.